The invention relates to blends of novolaks with cyanates and a process for their preparation.
In general, novolaks (phenol-formaldehyde poly-condensation products) can be cured thermally, namely both with and without catalysts. In order to ensure the dimensional stability of the cured components, they must generally be subjected to a post-cure treatment. In order to avoid warping, the parts have to be clamped in suitable moulds which means an additional expenditure for manufacturing.
The object of the invention was to prepare novolak blends which instead of having this disadvantage possess such dimensional stability already after short thermal curing that a free-standing conditioning is made possible.
It has been possible to achieve this object of the invention with the blends of novolaks with cyanates of the invention.
The blends are composed of a novolak of the general formula 
wherein R1 is hydrogen or methyl and n is a number greater than or equal to 1 and a cyanate of the general formula 
wherein R2 is methyl and m is a number greater than or equal to 1, and, optionally, one or more additional polymer(s), catalyst(s) and/or usual filler(s) or auxiliaries.
Novolaks of the general formula I, in particular those wherein R1xe2x95x90H, are commonly known oligomeric condensation products of phenols with formaldehyde, which are obtainable industrially in various grades and with various properties. They can be polymerized with and without additives. Suitable novolaks with R1=methyl (xe2x80x9ccresol novolaksxe2x80x9d) are, for example, available under the designation 371B from the company Borden Chemicals.
Cyanates of the general formula II are generally prepared by reacting the corresponding oligomeric phenols with cyanogen chloride in the presence of a tertiary amine on the industrial scale and are likewise available in various grades and with various properties. They can be polymerized with and without additives, too.
The compounds customarily employed in the processing on novolaks may be used as catalysts. These are known to those skilled in the art. Flame retardants or additives for improving the thermal conductivity or the mechanical or electrical properties of the cured resin may be added as fillers and auxiliaries, for example. These additives are also known to those skilled in the art.
In the blend of the invention, the cyanate fraction of the combined amount of cyanate (II) and novolak (I) is judiciously from 10 to 90% by weight and the novolak fraction judiciously from 90 to 10% by weight, corresponding to a weight ratio cyanate:novolak of 10:90 to 90:10.
In the production of the blends of the invention, the procedure in general is to mix the components physically. Mixing can, for example, take place in a suitable mill, for example in a vibrating mill; alternatively, the components can be melted together or combined with the aid of an appropriate solvent, in which latter case it is necessary to remove the solvent again.
The blends of the invention are thermally curable within a short time and are additionally notable for high long-term stability in the uncured state, as can be shown, for example, by the constant gel time.
A further advantage of the blends according to the invention is that the gel time and hence the curing cycle for the blend is considerably shorter than for the individual components of the blend. As a result, the use of catalysts becomes either completely superfluous, or can at least be reduced considerably.
The invention additionally provides the resin obtainable by thermal curing of the blend according to the invention.
The mixture is advantageously cured without a catalyst by heating it at a temperature from 100 to 300xc2x0 C., preferably from 150 to 250xc2x0 C., for a period of 1 to 15 minutes.
The resultant cured blends are notable for outstanding dimensional stability and can therefore be employed, for example, in binders for abrasives or in the electronics sector for chip encapsulations or printed circuit boards (PCBs)